PTR Records are normally maintained on your service providers DNS server because your public IP address block is part of their address space. All PTR records must be stored in a domain called: x.x.x.in-addr.arpa where the x's are your IP address block backwards. IE: Network 77.4.6.0/24 would be 6.4.77.in-addr.arpa and then PTR records would be created in that zone file with their host number mapped to the FQDN.

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Make sure your PTR and A records match. For every IP address, there
should be a matching PTR record in the in-addr.arpa domain. If a
host is multi-homed, (more than one IP address) make sure that all IP
addresses have a corresponding PTR record (not just the first one).
Failure to have matching PTR and A records can cause loss of Internet
services similar to not being registered in the DNS at all. Also,
PTR records must point back to a valid A record, not a alias defined
by a CNAME. It is highly recommended that you use some software
which automates this checking, or generate your DNS data from a
database which automatically creates consistent data.

If mailserver.company.com returns 123.456.789.123 then doing a reverse on that IP should return mailserver.company.com although you can get away with just company.com as the reverse most times it's best practice to have them match exactly.